lly reheating this hard form it gradually becomes
less brittle and softer, so that by regulating the temperature to which
steel is reheated in tempering almost any condition of temper demanded
for a given purpose, such as for making springs or cutting tools, can be
obtained.

~Steel alloys.~ It has been found that small quantities of a number of
different elements when alloyed with steel very much improve its quality
for certain purposes, each element having a somewhat different effect.
Among the elements most used in this connection are manganese, silicon,
chromium, nickel, tungsten, and molybdenum.

The usual method for adding these elements to the steel is to first
prepare a very rich alloy of iron with the element to be added, and then
add enough of this alloy to a large quantity of the steel to bring it to
the desired composition. A rich alloy of iron with manganese or silicon
can be prepared directly in a blast furnace, and is called
ferromanganese or ferrosilicon. Similar alloys of iron with the other
elements mentioned are made in an electric furnace by reducing the mixed
oxides with carbon.

~Pure iron.~ Perfectly pure iron is rarely prepared and is not adapted to
commercial uses. It can be made by reducing pure oxide of iron in a
current of hydrogen at a high temperature. Prepared in this way it
forms a black powder; when melted it forms a tin-white metal which is
less fusible and more malleable than wrought iron. It is easily acted
upon by moist air.

~Compounds of iron.~ Iron differs from the metals so far studied in that
it is able to form two series of compounds in which the iron has two
different valences. In the one series the iron is divalent and forms
compounds which in formulas and many chemical properties are similar to
the corresponding zinc compounds. It can also act as a trivalent metal,
and in this condition forms salts similar to those of aluminium. Those
compounds in which the iron is divalent are known as _ferrous_
compounds, while those in which it is trivalent are known as _ferric_.

~Oxides of iron.~ Iron forms several oxides. Ferrous oxide (FeO) is not
found in nature, but can be prepared artificially in the form of a black
powder which easily takes up oxygen, forming ferric oxide:

2FeO + O = Fe_{2}O_{3}.

Ferric oxide is the most abundant ore of iron and occurs in great
deposits, especially in the Lake Superior region. It is found in many
mineral varieties which vary in densi